The invention described herein relates generally to methods used in chemical machining, and more particularly to methods used for removing confining crucibles or molds from ingots and castings that are comprised of various oxide crystals including YAG and YAG based crystals, garnets, corundum crystals, and ceramic oxides, and that are prepared by either the Bridgman/Stockbarger melt growth technique or the permanent-mold casting method.
Chemical machining is the removal by chemical dissolution of material from exposed surfaces of a workpiece. Selective attack on different areas of the workpiece may be controlled by masking or partial immersion. Known chemical machining agents include solutions of sodium hydroxide for aluminum and solutions of hydrochloric and nitric acids for steel. Chemical machining is often used in the manufacture of solid state electronic components.
Melt growth is the best method for rapidly growing large single crystals of high perfection. The most straightforward and inexpensive melt growth technique is normal freezing, wherein a molten ingot contained in a crucible is gradually frozen from one end to the other. When this is achieved by the use of a two-zone furnace, it is called the Bridgman/Stockbarger method, which is described by D. C. Stockbarger in Reviews of Scientific Instruments 7, pages 133 to 136 (1936). The usual configuration is vertical with the melt in a crucible being lowered slowly from the hot zone to the cooler zone which is below the melting point. Petrosian et al, in U.S. Pat. No. 4,525,460 issued June 25, 1985 discuss the preparation of YAG or yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12) based crystals by the Bridgman/Stockbarger method, employing a molybdenum crucible or container, at column 2, line 30. In many instances using the Bridgman/Stockbarger technique, the crucible must be removed from the ingot by mechanical means, often with the aid of a lathe or a milling machine. A device for removing gasses from a melt within which monocrystals are being grown is taught by Tolksdorf in U.S. Pat. No. 3,677,712 issued July 18, 1972.
Unconfined crystals can be grown by the well known crystal pulling or Czochralski technique. Unfortunately, this method generally yields crystals that are relatively small and often difficult to control in shape. Various aspects of this technique are employed by Monchamp et al in U.S. Pat. No. 3,614,662 issued Oct. 19, 1971; by Takagi et al in U.S. Pat. No. 3,951,729 issued Apr. 20, 1976; and, by Auzel et al in U.S. Pat. No. 4,627,064 issued Dec. 2, 1986.
In permanent-mold casting, a fluid material is poured into a mold and allowed to freeze, to thereby provide a casting of the material. In many situations the casting can be ejected from the mold, and the mold re-used; however, that is not the case in situations within the purview of this application, wherein the mold must be machined away, for example by turning or milling, from the casting.
The methods of this invention are particularly advantageous with respect to the preparation of very hard and brittle crystalline oxide materials that can be easily broken and destroyed when subjected to the vibratory action of machinery when, in ingot or casting form, their enclosing crucible or mold is being machined away by turning or milling. This method is further particularly advantageous for removing crucibles and molds of complex shape which would be expensive, time consuming, or impossible to remove by conventional machining methods. These oxide crystal materials include, but are not limited to: YAG or yttrium aluminum garnet (Y.sub.3 Al.sub.5 O.sub.12) based crystals; garnets, which are silicates of the general chemical formula A.sub.3 B.sub.2 (SiO.sub.4).sub.3 where the A cations are dominantly Fe.sup.+2, Mn.sup.+2, Mg.sup.+2, and Ca.sup.+2 and the B cations are Al.sup.+3, Fe.sup.+3 and Cr.sup.+3 ; corundum crystals which are of the basic chemical formula Al.sub.2 O.sub.3 and include ruby, amethyst, emerald, topaz, and sapphire; and, ceramic oxides which are ceramic products composed of crystalline component oxides, which are well known in the prior art and are described, for example, in the textbook "Oxide Ceramics" by Eugene Ryshkewitch, Academic Press, New York and London (1960), which textbook is incorporated by reference herein. These hard and brittle oxide crystal materials are frequently prepared in crucibles or molds comprised of the group 5b and 6b metals, which are vanadium, niobium, tantalum, chromium, molybdenum, and tungsten, as well as in crucibles or molds comprised of rhenium or rhenium and tungsten alloy. These very high melting temperature metals can only be worked and machined with great difficulty. Consequently, the mechanical removal of these crucibles and molds is very time consuming, expensive, and labor intensive, and often results in severe damage or destruction to their included crystalline ingots and castings.